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Surface Treatments 191

ANTI-MIST COATING
The same reason can be attributed to anti-mist coating on the lens surface.
The property to prevent the formation of mist behind the lens is an
important feature.

TOUGHENING GLASS LENSES
Spectacle glass lenses can be strengthened by toughening process. The
finished lens is heated to its softening point and is followed by rapid cooling,
either by a stream of cold compressed air directed at the surfaces or by
submerging the lens in oil. This results in greater compressive strength in
the glass since outer region of the lens material cools more rapidly than the
interior, which remains hotter and more fluid for some time. When the
interior finally cools, it contracts and exerts tension on the rigid outer surface
to produce an envelop that encloses the interior under great tensile stress.
Toughened glasses are far more durable and more resistant to scratches
because of increased surface hardness. When they break, they shatter into
relatively harmless cubes of glasses.

Fig. 13.5: Birefringence pattern of thermally toughened lens
Glasses can also be toughened by a chemical process, in which a
compressive envelop, is produced by ion exchange as a result of hot dip of
the finished lens into a salt bath. Chemically toughened lenses are stronger
than air- quenched toughened lenses.

SURFACE TINTING OF LENSES
Tints are simply the methods of absorbing lights so that transmission is
reduced. If all wavelengths of light are equally absorbed, then a neutral